This invention relates to specialized aircraft having a hovering capability, and useful for applications such as aerial observation, aerial logging, etc.
A semi-buoyant composite aircraft for aerial logging is described in U.S. Pat. No. 5,082,205. The semi-buoyant aircraft comprises a large non-rotating balloon surrounded by a rotatable frame carrying propellers, and adjustable horizontal and vertical airfoils. A non-buoyant airborne Station Keeping Optical Observation Platform (SKOOP) also has been proposed as an aircraft having the capabilities of efficient hovering and precision station keeping. The SKOOP combines a conventional helicopter type main rotor with vertical airfoil surfaces mounted at the tip of each main rotor blade for direct force control in the horizontal plane to provide precision positioning capability, which cannot be achieved easily by a conventional helicopter.
A disadvantage of the hybrid aerial logging aircraft, and of the SKOOP, is that the vertical surfaces, along with the required bracing cables, substantially increase the aircraft's profile power losses.
This invention addresses the foregoing problems by an aircraft which combines two main rotors, preferably with one coned and the other drooped, positioned one above the other and joined at a radial station. This configuration provides a hovering aircraft with efficient direct horizontal and vertical force control without necessitating large pitch and roll attitude changes as with helicopters.